Vibration damping materials are widely used in automotive applications to dampen the vibration and sounds of road noise and the engine. Viscoelastic materials are commonly preferred as damping materials. A damping effect, linked to the energy dissipated in the viscoelastic material, increases with an increase of the Young modulus and of the loss factor of the viscoelastic material. Additionally, the damping effect also increases in relation to the subjected deformation of the damping material. Traditional materials with a very high loss factor but a low Young modulus provide limited damping to a metal plate, and are not effective dampers. Other materials with a high Young modulus but a relatively low loss factor (like rubber), are also not very good as dampers. Good damping materials incorporate a compromise between Young modulus and loss factor.
In viscoelastic materials, the Young modulus decreases with temperature and the loss factor has a peak in function of temperature. Therefore, there is a temperature window where the damping effect on a metal plate (called the composite damping) has a peak. Dampers are designed to have this peak fitted to the application temperature.
There are two main principles of damping: the free layer damping (FLD) and the constrained layer damping (CLD).
In FLD, the main deformation in the viscoelastic material is from strain caused by bending of the metal sheet, normally the structural part that is prone to vibration due to sound waves. The amount of energy dissipated increases with the thickness of the damping material. Therefore, traditional damping materials tend to be rather thick. Additionally, traditional damping materials have a rather high Young modulus. In CLD, the main deformation in the viscoelastic material is from shear, since the damping material is sandwiched between two stiffer plates (a structural base layer and an aluminum constraining layer) that are displaced along their planes during vibration. A thin layer of viscoelastic material can be used for CLD, since the shear deformation is much larger than the tensile deformation of the FLD. The damping material is also typically softer than in FLD to ensure that there is a big difference between the stiffness of the metal sheets and the damping effect, thus ensuring a high shear force. The fact that the deformation is very large compensates for the low Young modulus. If the material is too stiff, then the strain deformation is too low. Of course the damping effect increases with increase of the damping factor of the viscoelastic material.
Stiffer structures, like car body panels, require constrained-layer damping. Constrained-layer damping consists of a constrained damping material (3) between a structural base layer (4) and a constraining layer (2) (FIG. 1). Both the structural base layer (4) and the constraining layer (2) are effectively working as constraining layers. For constrained layer damping, the method of attaching the layers does not matter as long as adequate surface contact and coupling occurs. However adhesives (5) if needed should have high shear stiffness, as softer adhesives will not adequately transfer shear strain to the middle-damping layer.
The constrained layer damping material (3) and a metal sheet as the constraining layer (2) are applied to selected parts or areas of the vehicle, which form the base structural layer (4) of the constrained damping system. These layers prevent vibrations and noise from being transmitted inside the vehicle to the passenger compartment. However, many of the surfaces to be treated with this type of damping system are not horizontal, but instead are in the vertical direction, sloping or even up-side down. Therefore, the adhesive (5) must work against gravity and should have little or no slow creep or flow characteristics with age.
Typically, materials for the constrained layer dampening material (3) comprise of thermoplastic or rubber materials, which are capable of suppressing vibrations and sounds. Bitumen based damping material is cheaper than rubber based material but has the disadvantage that it is not auto adhesive.
Bitumen materials are normally characterized by a combination of penetration, softening point and viscosity, and can be divided in the following classes of material: penetration grade bitumen, oxidized bitumen, and hard grade bitumen.
Penetration grade bitumen is obtained from fractional distillation of crude oil. The lightest fraction are vapours i.e. butane and propane, while the heavier fractions are taken off the column for gasoline production. The heavier fractions include kerosene, then gas oil, and high molecular weight hydrocarbons as the heaviest fraction. These heavy hydrocarbons, which are called long residue, are further distilled in a vacuum distillation column to produce gas oil, distillates, and short residue. The short residue is the feed stock for producing over 20 grades of bitumen. These are classified by their penetration index, typically PEN values of 10 to 330 dmm, which is the distance in tenth millimeters that a needle penetrates the bitumen under a standard test method. Penetration grade bitumen are characterized by penetration and softening point.
Oxidized bitumen are characterized by softening point and penetration, eg 85/40 is oxidized bitumen with a softening point of 85 and a penetration of 40. Oxidized bitumen is obtained by further processing the short residues. They are semi blown or fully blown with air to increase their molecular weight.
Hard grade bitumen are characterized by their softening point and penetration, but are only designated by the softening point range, e.g. H80/90.
Experience has shown that, in sound deadening panels in vehicles, penetration or PEN grades typically provide better damping characteristics than oxidized grades. However it is common belief that 15-pen is to brittle, whereas 50-pen and higher pen grades are too soft, producing panels that flow during storage. The bitumen is positioned on the vehicle body panels to be dampened and subjected to heat (around 140° C.), whereupon they conform to the shape of the body panel under their own weight and adhere strongly. Typical composition is (% of mass):                Bitumen (25 pen) 25-30%        Polymer (e.g. APP, EVA etc) 0-5%        Fibre 3-5%        Filler (e.g. Jimestone, clay, mica etc) 60-70%. Morgan et al., Shell Bitumen Industrial Handbook, Thomas Telford, 1995.        
A common method of applying vibration damping material (3) is to provide it in the form of a sheet or tape, which includes an adhesive layer (5). The adhesive layer (5) adheres the damping material (3) to the desired substrate, such as an automobile body panel or interior panel. However this requires the vibration damping material (3) be laminated to at least one pressure sensitive adhesive layer, or heat activated adhesive layers.
It is well known that the optimal time for installing these damping sheets is prior to curing the exterior surface paint of the vehicle, which typically occurs in a bake oven that can reach temperatures in excess of 190° C. As such, the ability of the adhering material of the sheet must be able to withstand such intense temporary temperatures. It has been found that thermoplastic resin based adhesives do not provide adequate adhesive strength after the heat bake process due to loss of mechanical properties as temperatures increase.
As such it is well known that heat activated adhesives, such as thermosetting adhesives, are preferred wherein the adhesive's activation point can be appropriately modified, for example, based upon the temperature of the heat bake process. Such adhesives thus have the ability of withstanding the temperatures typically found in the heat bake process, while maintaining the requisite adhesive strength. However, the damping material laminate must be temporarily attached to the metallic medium, such as the automobile body panel or interior panel, in the interim until the adhesive is properly activated.
U.S. Pat. No. 3,243,374 discloses the use of a ferrite powder within a bitumen-based noise and vibration-damping sheet to allow preliminary positioning of the sheet during assembly together with a heat activated adhesive. At application and before the adhesive is heat activated, the sheet is held on in place magnetically.
An alternative to thermosetting adhesives is the use of an auto adhesive damping material. Currently, there are two main materials on the market, one material composite based on bitumen and one based on rubber.
Constrained layer damping material can be made of a combination of bitumen and a tackifier, for example hydrocarbon resin to obtain an auto adhesive damping material. Normally two grades of bitumen, a 20/30 pen grade bitumen, which has a penetration between 20-30 dmm and a ring & ball softening around 55° C., and a blown bitumen, with a penetration between 35-40 dmm and a ring & ball softening around 100° C., produce the damping material. During manufacturing, the two different bitumen materials are stored in separate tanks and mixed together during the preparation of the damping material, in order to increase the melting temperature of the final material. However, the blown bitumen is a niche product for big oil companies, and therefore it is increasingly difficult to obtain the blown bitumen material to make the dampers. Bitumen with hydrocarbon resin, can be used without the need for curing as the resin is already mixed in the bitumen. However, hydrocarbon resin is no longer allowed inside the passenger area of the car due to its high level of VOC (Volatile organic compounds).
U.S. Pat. No. 6,828,020 discloses an example of a butyl rubber type of damping material. Butyl rubber constrained layer material was developed for the building industry and is now commonly used in the damping of automotive body panels. However, the production and raw materials for butyl rubber are expensive. In particular, the mixing of the ingredients requires a high shear stress mixer and is therefore an energy consuming process. Currently, butyl rubber material developed for the building industry is also used for vehicle applications, but not all auto adhesive butyl rubber material is able to withstand the required curing temperatures. For example, the aluminum foil of the constraining layer (2) tends to delaminate or start sliding from the damping material (3) during the curing process. The butyl damping material currently on the market for automotive applications have partly been modified in order to support a curing temperature of 190° C.
These self-adhesive or auto-adhesive damping materials are produced in sheet form with an aluminum foil as a constraining layer on one side and a release paper on the opposite side of the sheet. After removing the release paper, the adhesive material of the damping material is directly applied to the panel to be dampened, without the need of additional adhesive layers.